1. Field of the Invention
This invention relates to a turbo-molecular pump, especially to a turbo-molecular pump having helical grooves formed on the circumference of the rotor and stator.
2. Prior Arts
A conventional turbo-molecular pump has a plurality of blades which are disposed on circumference of the rotor and stator in plural stages in a manner that the blades of the rotor and stator are alternately arranged along a rotary axis of rotor.
But, in the exemplified case of a pump with only blades, the pump compression ratio is suddenly reduced below the more than 10.sup.-3 torr suction-port pressure region and the motor load for driving the rotor is suddenly increased.
Therefore, a pump disclosed in Japanese Patent Publication No. 33446/72 is formed of a thread-groove (helical groove) on either the rotor circumference or the stator circumference at the discharge-port side of the blades. Further, a pump disclosed in Japanese Patent Provisional Publication No. 182394/85 is formed with thread grooves on both the rotor circumference and stator circumference at discharge-port side of the blades so that thread-grooves are reversely threaded relative to each other in the threading direction.
Regretfully, the former pump disclosed in Publication No. 33446/72 has a defect that the pump compression ratio is so suddenly deterionated in the more than 1 torr region that sufficient compression ratio is not attained, as shown by the characteristic curve 100 of FIG. 2. The latter pump disclosed in Provisional Publication No. 182394/85 has a defect that a sufficient compression ratio is not obtained under the pressure region from ultra-high vacuum to 1 torr, as shown by the characteristic curve 102 of FIG. 2.